In recent years, from growing awareness of the global environmental problem, automakers are promoting the weight reduction of a car body with the aim of improving fuel consumption. In addition, from the viewpoint of the safety of a passenger, the collision safety standard of an automobile is tightened and the durability of a member against impact is also required. Consequently, the percentage of a high-strength steel sheet used in an automobile further increases recently and a high-strength hot-dip galvanized steel sheet is proactively applied for body frame members and reinforce members requiring rust preventive performance. Required properties become more advanced in accordance with the expansion of the application of the high-strength steel sheet and the improvement of the workability of a base material is strongly demanded in the case of a less-formable member.
A material developed as having both strength and workability is a dual phase steel sheet (hereunder referred to as DP steel sheet occasionally) mainly composed of ferrite and martensite. In JP-A Nos. 122820/S55 and 220641/2001 for example, a high-strength galvanized steel sheet excellent in balance between strength and elongation and the production method thereof are disclosed. In the meantime, together with the workability, energy absorbability at collision is required and a high yield strength, namely a high yield ratio, is also important in the case of a high-strength steel sheet for a body frame. In JP-A No. 322539/2002 for example, a steel sheet that makes use of precipitation particles, thus has a high yield strength, and is excellent in workability is disclosed.
In the technologies disclosed in JP-A Nos. 122820/S55 and 220641/2001 however, martensite is generated at the cooling process after galvanizing or after succeeding alloying treatment, mobile dislocations are introduced in ferrite during the cooling process, and consequently the yield strength lowers. Further, in the case of JP-A No. 322539/2002 where the yield strength is enhanced, precipitation particles of a nano level are used, but it is difficult to disperse the precipitation particles finely when annealing is applied after hot rolling or cold rolling, and thus it is also difficult to obtain both a high yield strength and a high ductility simultaneously.
In addition, a high-strength hot-dip galvanized steel sheet having both good spot weldability and a high yield ratio and the production method thereof are disclosed in JP-A No. 274378/2006. The hot-dip galvanized steel sheet however contains elongated crystal grains having an aspect ratio of three or more in the metallographic structure and thus is nonuniform structurally, and hence good workability is hardly obtainable.